Desecration! In a ferocious parting blast, the outgoing chairman of the National Trust accuses an arrogant and philistine political class of ruining some of our loveliest countryside

> Outgoing National Trust chairman said English countryside is threatenedSimon Jenkins said we should grade countryside as with historic houses > Farmland has been replaced by warehouses, estates and wind turbines > HS2 railway line will crash through the Chilterns and parts of Derbyshire

What do the Royal Family, Shakespeare and the English countryside have in common?

The answer is they are 'things about Britain' most treasured by the British people in polls at the time of the Millennium. To threaten any of them would be unthinkable.

After six years chairing the National Trust — from which I retired last week — I am left with one gloomy fact: one of these treasures is indeed threatened. The English countryside.

It is not about to disappear, covering as it does some 80 per cent of England's land area, but its fate can be clearly seen by anyone driving up the M1 through the East Midlands. Rolling farmland is replaced by warehouses, bleak housing estates, wind turbines and advertising hoardings in fields. It is the start of the 'tat' that is familiar the world over when planning control collapses.I am sure politicians such as David Cameron, George Osborne, Ed Miliband and Nick Clegg never rose one morning and declared: 'This is just how I want England to look.' They have probably not noticed. They holiday in 'unspoilt' places at home or abroad, and see England only from train and car windows. To them, the countryside is where poor people spend their leisure, sitting in traffic jams. They have come to treat it as a dustbin for energy policy, industry policy, housing policy and transport policy.

The 1947 Town & Country Planning Act ranked with the post-war NHS as a work of British genius. The United Kingdom was, with Holland, the most densely populated country in Europe. The whole of the South-East faced suburbanisation, like much of New Jersey in the U.S. or Germany's Ruhr valley. The Act set out clear boundaries between town and country. There should be belts of green around crowded cities. Elsewhere, country should remain country, not pockmarked with random buildings like most of Europe. It worked. New areas of settlement were planned as such. In the countryside, regulation confined buildings to existing plots. Even when land the size of Bristol was going under concrete each year, Britons could look out even over parts of crowded Surrey, Cheshire and Yorkshire and see landscape that our Victorian forebears would have recognised.

There are still pristine rural views from the North Downs over the Weald, from Malvern over the Wye, and from the Dales over the Vale of York. These spectacles are being drastically altered. The Severn valley from the Cotswolds is blotched with warehouses and tower blocks. Yorkshire's once-glorious Calder and Aire valleys, 'lost' between the Peak District and the Yorkshire Dales, are becoming a shambles of ill-sited housing estates and pylons.

And then, of course, there is the plan for the HS2 railway line, surely Britain's craziest infrastructure project, which will crash through the Chilterns and pass beneath the terrace at the National Trust's Hardwick Hall in Derbyshire.

The Northumbrian coast is wild no more, raddled with lines of wind turbines. Giant turbines are rising round the perimeter of the Lake District and on the exquisite Lleyn peninsula beyond Snowdonia. Unbelievably, the glorious county of Dorset is to be desecrated by three huge turbine arrays along its spine. Is this how David Cameron wants his Coalition to be immortalised?

In January 2012, Cameron famously sat in a rustic kitchen and told the BBC's Countryfile that 'I would no more put [the countryside] at risk than I would risk my own family'. There would be no change to green belts and no large housing estates 'plonked down' next to villages. Planning, said Cameron, would revert to 'local communities', who would have the power to decide where buildings should go.

What did Cameron mean? He must have known his ministers were proposing the precise opposite. He was already pledging the riches of Croesus to landowners — including his own father-in-law — to erect turbines across the countryside. Had Cameron been Thatcher, his antennae would have been alert to the planning drafts passing between lobbyists and Eric Pickles's local government department. Whitehall policy-making was being drafted by groups such as the Home Builders' Federation, the Country Land and Business Association and the British Property Federation.

Simon Jenkins said we should grade the English countryside as with historic houses

Old land-use plans were to be torn up and replaced with new ones, demanding 20 per cent more building land from areas failing to meet Government targets. Communities had to be consulted, but if a council did not offer an alternative to the submitted plans by a due date — and half were not — any planning application would be approved if deemed 'sustainable'. This vague word was soon defined as 'profitable'.

The 2011 planning draft was a builder's charter. Protests from the National Trust, the Campaign for the Protection of Rural England (CPRE) and others secured some changes. Eric Pickles conceded some priority for green belts, and a shift towards using brown-field land (previously used for industrial or commercial purposes).

However, the readiness of Whitehall to overrule local wishes — a key demand of the builders' lobbyists — remained in place, and has been gushing gold ever since. This was not planning, but a declaration of war. Villages and towns turned in desperation to lawyers, clogging the planning courts. Landowners rushed in with planning applications lest the rules changed. Civil servants were reputedly told by Osborne's Treasury to be 'deaf to the Nimbys'.

The impact was instant. Every planning appeal bar one within the Cotswolds 'area of outstanding natural beauty' was allowed. Howls of pain came not just from such once-protected gems as Stow-on-the-Wold, Chipping Camden and Tetbury. There were applications for 2,000 sites outside Lincoln, and 900 under the walls of ancient Warwick. Winchester was put under siege, as was Sandbach in Cheshire, Newmarket and Ashford in Kent. Twenty-three acres of green belt were to be colonised around Durham — in a county thick with empty properties. Notifications of contested plans adjacent to National Trust property soared to 400 a year.

Under Labour, there was a peak of roughly 80 per cent of new development being on brownfield sites. This has declined to little more than 50 per cent. Even as the supermarket boom collapsed, Pickles was directing three-quarters of new retail building to be 'out of town'. The result? Ever-more deserted High Streets and rows of boarded-up shops. British planning had reverted to the Dark Ages.

The countryside's lack of political muscle and influence remains curious — as does Westminster's disregard for it. The old primacy of agriculture has collapsed. City-dwellers now constitute the majority of voters. They seem careless of important issues such as food security, bird conservation, ash tree die-back and bovine TB, which leads to tens of thousands of cattle being killed a year. Perhaps that's why the 2012 planning changes seriously worried Downing Street only when Ukip came out against them, and against wind turbine subsidies. It was as if the Tories — traditionally seen as the party of the countryside — had simply given up on its rural roots.

It may be, as the Prince of Wales writes in this week's Country Life, that most Britons are now 'four or more generations from anyone who actually worked on the land'. It may just be that money talks. Owners and developers were making huge profits from standard 300-unit housing lots and could afford to appeal against every rejection.

More surprising was the lack of support from the Left. Much of the original pressure to protect the British landscape had come from groups such as ramblers, cyclists and the National Trust. Ed Miliband uttered no word of protest at the planning changes, and indeed promised to overrule councils that obstructed developers.

Nothing has so blighted this debate as the pretence that rural planning has caused the 'housing crisis'. The idea that a shortage of urban housing can be solved in the green fields of Middle England is absurd. Yet developers — and many politicians — imply that anyone who defends rural Britain is being callous towards the homeless.

It is a total fiction that there is 'no land left' in existing settlements, and that only the countryside will do. British cities are embarrassingly behind the rest of Europe in post-industrial renewal. An estimated 1.5 million sites allotted to housing lie unused in urban areas. Whitehall figures suggest an estimated one million homes are empty — and this is probably a gross under-estimate.

Nor are these just in the North and Midlands. The property agents Stirling Ackroyd recently declared there was space for 550,000 new houses in crowded London alone. I have visited every corner of England in my time at the National Trust. Nothing was so depressing as to drive across miles of urban dereliction, boarded up high streets and vandalised housing estates, only to emerge into the countryside and bulldozed fields and huge Bovis, Persimmon and Barratt estates. Such settlement sprawl means long commutes. It congests roads and demands investment in new shops, schools and hospitals.

We should be increasing urban densities, not reducing them. Most British town centres are still built to a height of three to four storeys. Most European towns are seven to eight storeys. This is unsustainable waste.

Rural Britain does not hold the key to housing policy, only to housing profit for developers. The fierce defence of their environs by those who live in the countryside can seem selfish. But the job of planning is not to dictate who lives where: it is to guard the public interest. The public clearly values the rural landscape and wants it protected for its enjoyment. But what is true of the countryside is true of land in general. As Mark Twain said: 'They are not making it any more.'

For the past 50 years, we have relied on a political consensus to guard rural Britain. It was sacrificed only by some overriding national good, such as a new town, motorway or airport. Since the Coalition's deregulation, that has no longer applied. Villages are at the mercy of landowners, much as in Sicily, Greece or Spain. Ministers claim to have 'empowered' local people to stop this, but in many cases they cannot. If nothing changes, the result will be a rural Britain confined to national parks and National Trust properties.

My solution would be to grade the countryside as we do historic buildings. The National Grid last week proposed to bury its pylons where they spoil valued views. Such awareness of the visual quality of the landscape is a shaft of light on a gloomy scene. 'Listing the landscape' would be no great task, as every field is on a Whitehall computer. It would protect for all time the bulk of the countryside, whose scenic qualities Britons clearly want preserved. It could also free for development tens of thousands of acres of land of no landscape value. This might relieve at least some of the current pressure on green belts. Yes, these areas need some revision, but at present no one trusts any minister to reassign the belts, knowing that the slightest relaxation would just see them vanish.

We must hope that one day Britain can recover from this philistinism, that it can recapture its ability to value rural landscape. At present, defending the countryside is a lonely business. When I hear Government ministers and lobbyists abusing those defenders as Nimbys, I can only raise a glass and cry: 'God stand up for Nimbys.'